NetworkPolicy receives scant mention in the docs

[kingdonb]

There are only a few mentions of NetworkPolicy resources in the docs, and none of them which are searchable appear to be particularly comprehensive on the topic.

https://fluxcd.io/docs/use-cases/azure/#cni-and-network-policy https://fluxcd.io/docs/installation/#bootstrap-with-terraform

Neither of these are general information about network policies and neither of them explain why network policies are important for Flux, (or why we have these network policies in particular, or what they are specifically intended to do.)

I built some docs about NetworkPolicy into the Flux E2E guide that is in #725 but they do not belong there, should live somewhere permanent.

I am opening this issue so that the E2E guide can be merged without losing the reference to another improvement that was out of scope. I suspect that as we review the doc, we're going to do this again with other sections that are more detailed in the E2E doc than we'd like to see, for evergreen and maintainability of the E2E doc, but yet still not detailed enough anywhere else in the docs.